1. Field of the Invention
The present invention relates generally to cryogenic systems and more particularly to a cryogenic system for cooling selected surface areas in a safe, carefully controlled manner. While the highly versatile system of the present invention has many non-medical uses in supercooling selected surface areas, in cryo-adhesion, cryo-fragmentation, cryo-solidification of liquids, the like, and an important application is in the medical field of cryosurgery.
2. Prior Art
The term "cryosurgery" relates broadly to a wide variety of surgical procedures wherein tissues are selectively destroyed by freezing with cryogenic materials.
While cold temperatures were reportedly used as early as the 1850's in the treatment of such diseaases as cancer, significant medical use of cryogens did not occur until the 1890's. Processes for liquifying air were developed as early as 1877, but the new product did not come into broad use until the vacuum insulated cryogen storage vessel was developed nearly 15 years later.
Liquid air continued to be the principal liquified gas cryogen in medical use until the 1940's when liquid nitrogen become available. Liquid nitrogen is preferable to liquid air and most other liquified gases for medical uses because it does not support combustion. It continues to be the preferred cryogen for most medical purposes not only because it is available very economically, but also due to its very high latent heat of vaporization. Whereas, other available non-flammable cryogens such as CO.sub.2, N.sub.2 O and freon change phase at -90.degree. centigrade or above, liquid nitrogen changes phase at -196.degree. centigrade. The resulting extraordinary latent heat of vaporization makes liquid nitrogen of unique value in effecting a fast deep penetrating freeze in diseased tissues.
Four cryogenic treatment techniques have been used with success:
(1) The swab method;
(2) The spray method;
(3) The open probe method; and
(4) The closed chamber probe method.
The swab method utilizes a swab such as a wooden stick with cotton tightly twisted around one end to form an absorbent tip. The cotton covered tip is dipped in a cryogen and then is placed on the tissue to be treated. This direct application technique is not well adapted for use with liquid nitrogen to treat large tissue areas or to effect deep penetration freezing.
The spray method is another direct application technique where a cryogen mist is sprayed onto the tissue to be treated. This technique is commonly used with liquid nitrogen to thinly freeze broad surface areas. It can also be used to obtain substantial depth of penetration with prolonged application. The spray method has several drawbacks including difficulty in achieving uniformity of penetration, and in controlling the deflection of cryogen droplets from the primary target. These drawbacks render the spray technique unsuitable for intra-cavity use and for discrete site freezing-in-depth to controlled limits.
The open probe method utilizes a tubular probe having an open applicator end which is pressed into surrounding engagement with a tissue surface to be treated. The tissue closes the applicator end and cooperates with the tubular wall of the probe to define a chamber into which liquid cryogen is introduced for direct application to the tissue. During cryogen application, the tissue freezes firmly to the end of the probe.
The closed chamber probe method typically provides a probe structure having a relatively rigid tip cooled internally by cryogenic exposure. It differs from the swab, spray, and open probe methods, in that the cryogen itself is not brought into contact with the tissue to be treated. This better enables the probe to be used in intra-cavity applications. Sufficient accuracy is attainable to enable discrete site freezing-in-depth. The closed chamber probe has even been used in micro-surgery.
The freezing process is typically monitored by carefully observing its progress as indicated by changes of color and texture of tissues. Needle-like thermistor units implanted in the tissues to be frozen are also used on occasion to provide a more exact indication of temperature changes as they occur.
The depth, extent and rate of freezing achieved with any cryoprobe system depends on several factors, including the type of cryogen used, the area of contact between the cyroprobe and the target tissue, and the time during which the tissue is exposed to cryogen cooling. Other factors which also come into play include the rate at which cryogen is supplied to the probe tip, and the efficiency of the probe in transferring heat out of the tissue and into the cryogen. Since the target tissues freeze rigidly to the probe tip during treatment and the probe-to-tissue contact cannot be disrupted reliably without thawing, the freezing process is controllable chiefly by regulating the supply of cryogen to the probe tip.
Early cryoprobes used in the 1900's included cylindrical and spherical applicators made of glass or brass which were filled with liquid and rolled over to the tissues to be treated. More sophisticated probes were developed together with more precise instrumentation in the early 1960's through the work of Dr. I. S. Cooper. The Cooper system employed a cannula about 2.2 mm. in diameter which was vacuum insulated except for the tip. The temperature of the tip was monitored by a thermocouple control system which regulated the flow of liquid nitrogen to the cannula.
While various types of control systems are known for regulating the flow of cryogen to the tip of a probe, most of these known systems have valves or other controls located at some distance from the site of freezing. Adjusting the flow of cryogen during surgery accordingly requires that apparatus outside the surgical field be manipulated. This is undesirable not only because it is often inefficient and clumsy, but also because time delays are involved in effecting the adjustments and before uniform cryogen flow is re-established at the newly adjusted setting.
Still another problem with known and proposed cryoprobes is that they typically employ relatively rigid tips or applicator surfaces which rarely conform with the tissue surfaces to be treated. The accepted solution has been to design a wide range of specialized probe configurations and to select the most appropriate available probe for surgical use. Maintaining a large probe inventory is a costly undertaking which is often frustrated by non-conformance of available probe shapes to the highly irregular and randomly variable shapes of tissue surfaces.
Most known probes cannot be reshaped due to the rigid nature of the materials from which they are constructed. Most probes are also provided with directed cryogen flow paths which become disrupted if the probe is deformed, thereby rendering the probe unusable.
The use of a thin, softly compliant sleeve or membrane supported on a hard foamed plastic or metallic probe body is proposed in U.S. Pat. No. 3,421,508 issued Jan. 14, 1969, to F. L. Nestrock. The applications to which the Nestrock probe can be put are severely limited by the relatively rigid nature of the probe. It cannot be reshaped by hand prior to use. The resilience of the probe is limited to the resilience of the outer membrane or sleeve, and as such is not readily deformable to accommodate substantial tissue surface irregularities. The cryogen flow path through the probe tip requires a remotely controlled flow which is dependent upon the cryogen supply system design. The need for a carefully controlled cryogen flow through the probe severely limits the selection of probe sizes and shapes which can be used without redesigning the cryogen supply system. Moreover, the multi-part precision machined nature of the probe assembly result in a very costly medical apparatus.